Lore

The Brain of Mensis inhabits the Nightmare of Mensis.

The immense brain that Mensis retrieved from the nightmare
was indeed lined with eyes on the inside, but they were of
an evil sort, and the brain itself was terribly rotten.
But even still, it was a legitimate Great One, and left a relic.
A living relic, at that, which is a precious thing indeed.

Based on the fact that the Brain does not cause frenzy build up and has had metal pegs hammered into it in order to secure chains, the Brain appears to have been lifted out of the abyss like environment from which it can be dropped into. This also makes sense considering the Brain is said to have been "retrieved" from the Nightmare.

Theories

The Brain of Mensis comes from all of the School of Mensis' brains combining into one

Sources

Living String
Special material used in a Holy Chalice ritual.
The immense brain that Mensis retrieved from the nightmare
was indeed lined with eyes on the inside, but they were of
an evil sort, and the brain itself was terribly rotten.
But even still, it was a legitimate Great One, and left a relic.
A living relic, at that, which is a precious thing indeed.

Iron Door Key
Of the three great bridges that link the two cathedral
cradles, this key opens the iron door that leads to the
midlevel bridge.
There are no thieves in the nightmare. Then, why lock a
door? Be warned, there must be a very, very good reason.

Moon Rune
A secret symbol left by Caryll, runesmith of Byrgenwerth.
A transcription of "moon," as spoken by the Great Ones
inhabiting the nightmare.
Gain more Blood Echoes.
The Great Ones that inhabit the nightmare are
sympathetic in spirit, and often answer when called upon.

One Third Umbilical CordDescription 4
A great relic, also known as the Cord of the Eye. Every
infant Great One has this precursor to the umbilical cord.
Every Great One loses its child, and then yearns for a
surrogate. This Cord granted Mensis audience with Mergo,
but resulted in the stillbirth of their brains.
Use to gain Insight and, so they say, eyes on the inside,
although no one remembers what that truly entails.